Going back to work after the birth of any child is difficult for a mother...at least that's what they say on the Internet! My thought is that the level of difficulty depends on the mother and her individual situation. For me, going back wasn't hard, but the timing could have been better. I would be back for the last six weeks of school and I was walking right into spring concert season, which meant three weeks of after school rehearsals and evening performances.
I think that the fact that the twins were in the hospital for so long made it easier for me to go back to work. I had no choice but to get used to leaving them in the NICU every day. Going to work was a piece of cake, emotionally speaking. The work load was nuts, and I had stuff to submit from when I was out, but I had all of the paperwork under control within a week.
Now I just had to get through the 16 hour days that were performance days while taking care of the twins. Naturally, they both decided to start sleeping through the night the week AFTER my concerts were done!